The present application relates to an optical device, a method of manufacturing the same, a replica substrate for producing the optical device, and a method of producing the same. More specifically, the present application relates to an optical device having a plurality of structures each composed of protuberance or pit provided on a base surface.
In the field of optical devices using transparent substrates such as those made of glass and plastic, there has been known a technique of reducing light reflected on the surface, and of improving the transmission characteristics, such as forming a fine and dense pits and protuberances (sub-wavelength structure; “moth-eye” structure) on the surfaces of optical devices. When the sub-wavelength structure, having a periodicity of as large as wavelength of visible light or around, is provided to the surface of optical devices, light passing therethrough is generally diffracted, and thereby the transmitted light largely reduces straightly-propagating components. On the contrary, no diffraction would occur when the pitch of the sub-wavelength structure is shorter than the wavelength of light to be transmitted therethrough. For an exemplary case where each sub-wavelength structure is composed of a cone having a depressed cone surface, an effective anti-reflective effect and excellent transmission characteristics may be obtained with respect to light having a single wavelength corresponded to the pitch and depth thereof.
In the paragraphs below, a shape characterized by a cone surface depressed in a form of pit will be referred to as “tent-form”, and a shape characterized by a swelled cone surface in a form of protuberance will be referred to as “bell-form” or “bell-type”.
The optical device having the above-described, tent-form sub-wavelength structures may be manufactured as follows. First, a pattern of pits and protuberances is formed on a photoresist on a Si substrate by electron beam recording, and the Si substrate is then etched through the pits and protuberances photoresist pattern used as a mask. As a result, tent-form sub-wavelength microstructures (approximately 300 nm in pitch, approximately 400 nm in depth) are formed on the surface of the substrate, and whereby a Si master disc is formed (see FIG. 18). The microstructures are provided to a tetragonal lattice pattern or a hexagonal lattice pattern.
In the Si master disc produced as described above, anti-reflective effect may be obtained over a wide range of wavelength of light. In particular, for an exemplary case where tent-form sub-wavelength microstructures provided according to the hexagonal lattice pattern as shown in FIG. 19, an advanced anti-reflective effect (reflectivity of 1% or lower) may be obtained over the visible region (see FIG. 20). Reference numerals I1, I2 in FIG. 20 represent reflectivities of flat portion and patterned portion of the Si master disc, respectively.
Next, a Ni-plated stamper of the manufactured Si master disc is manufactured (see FIG. 21). As shown in FIG. 22, on the surface of the stamper, pits and protuberances patterned which is complementary to those on the Si master disc are formed. Next, the pits and protuberances pattern are transferred to a transparent resin such as polycarbonate using the stamper. By this process, a desired optical device (replica substrate) may be obtained. Also this optical device may successfully achieve an advanced anti-reflective effect (reflectivity of 0.3% or below) (see FIG. 23). Reference numerals I3, I4 in FIG. 23 represent reflectivity measured in the absence of the pattern, and in the presence of the pattern, respectively.
The optical device having the tent-form sub-wavelength, however, suffers from a problem in that the reflectivity increases in the longer wavelength region (700 nm to 800 nm), as shown in FIG. 23.